As the extension of the broadband services to the family, the focus of the construction of the communication optical network is developing from the core network to the optical fiber local area network, access network and even the Fiber To The Home (FTTH). FTTH, as the last mile of the access network, is affected by the complex applications (mostly the streets, buildings, corners, etc.). Moreover, since it has many access points and it is affected by the hanging, pulling, bending and other factors in the wire arrangement, the fiber has lots of bending phenomenon, which proposes a higher demand on the bending performance of the fiber. The mode field diameter of the conventional G.652 fiber is about 9.2 μm. Due to the influence of the bending loss and the mechanical stress, it is generally required that the bending radius of the fiber in the use shall be not less than 30 mm, which is hard to wiring along the indoor walls and coil the remaining length of the fiber like the copper wire. The popularity of the FTTH needs to further improve the operating performance of the fiber and make the miniaturization of the indoor devices and the junction box. Therefore, the bending insensitive fiber will become the focus of the fiber application in the future.
The International Telecommunication Union (ITU) officially introduced the specifications and standards of the bending insensitive fiber cable in December, 2006 and defined such fiber cable as G.657 fiber cable. The G.657 fiber cable is mainly characterized in the fiber can be equipped with a smaller bending diameter and is of lower bending loss and able to better meet the construction requirements of FTTH project. Compared with the conventional single mode fiber (C-SMF), G.657 fiber has better bending performance to ensure the fiber's access to the network, including the various kinds of wiring in the constructions of the fiber access network terminal. According to the operating wavelength and the use scope, G.657 fiber can be divided into two types: G.657A and G.657B. G.657 A is characterized in, under the premise of fully meeting the specifications of G.652D, improving the bending loss feature of the fiber, which can work in the entire operating wavelength range of 1260-1625 nm. G.657A fiber can be regarded as a subclass of G.652D, with the transmission and interconnecting performance the same as G.652D. However, G.657A fiber has better bending performance and more accurate geometry requirements. G.657 B is characterized in the overall improvement of the bending loss feature of the fiber. Due to primary usage in buildings, it is not necessary to meet the specifications of G.652D. The transmission operating wavelengths of G.657B fiber are respectively 1310 nm, 1550 nm and 1.625 nm. Due to the smaller mode field diameter, G.657B fiber is also different from G.652 in respect of the splicing and connection features. But G.657B fiber can work normally in the circumstance that the bending radius is very small.
In fact, the bend insensitive fiber is always one of the important parts of the development and research of the fiber materials. In the 80s of last century, AT&T in the United States and NTT in Japan respectively introduced the depress cladding and convex refractive index fiber, the cut-off wavelength of which operated at 1.3 μm can be made up to 1.35 μm. Through decades of efforts, today many companies have already launched the bend insensitive fibers which the bend insensitive performance has been improved greatly. For example, FutureGuide-SR15 and FutureGuide-SR15E bend insensitive fiber of Fujikura Ltd. respectively correspond to ITU-T G.652.B fiber and ITU-T G.652.D fiber, the minimum allowable bending radiuses of which are up to 15 mm, which has been reduced by half by comparing with 30 mm bending radius of the conventional single mode fiber. The minimum allowable bending radius of PureAccess-Ultra fiber introduced by Sumitomo Electric has been reduced from the conventional 30 mm to 7.5 mm. For the bend insensitive fiber of Corning, when the bending radius is 32 mm, its loss at 1550 nm is not more than 1 dB and the corresponding cut-off wavelength is between 870 nm-970 nm. The loss of the bend insensitive fiber of FiberHome at 1550 nm is not more than 0.5 dB and the corresponding cut-off wavelength is below 1290 nm under the bending radius of 10 mm and 30 turns.
The improvement of the bending performance of the fiber can be started from the improvement of the fiber structure. For example, for the hole assisted bending insensitive fiber of Fujikura (China Patent Application No.: 200580022430.2, Publication No.: CN1981223A), the bending loss with the 5 mm bending radius at 1550 nm wavelength is 0.012 dB/turn, the mode field diameter (1550 nm) is 7.8 μm and the cut-off wavelength is 1.28 μm. Although the mode field diameter of the fiber is slightly smaller than C-SMF, the attenuation of the fiber can also reach the level of C-SMF when making the hole assisted fiber matching with the C-SMF cross section through the electric arc so that the average splice loss can be up to 0.05 dB. And the attenuations at 1.30 μm and 1.55 μm are 0.50 dB/km and 0.28 dB/km. A kind of air hole assisted fiber (Application No.: 200610119574.6, Publication No.: CN1971323A) has been also disclosed. The trench assisted BendBrightXS fiber of Drake does not only meets G.657A standard (completely compatible with G.652D) but also meets G.657B standard (the bending loss is smaller). The minimum bending radius is between 1-10 mm. Moreover, the average splicing loss is smaller than 0.05 dB when using a suitable procedure splicing with C-SMF. ClearCurve™ fiber, a new bending insensitive fiber based on the nanoStructures™ technology launched by Corning in the United States, is not only completely compatible with G.652D, but also the bending insensitive property is 10 times higher than the specified value of G.657B. The European patent application (Application No.: 89104889.4 and Publication No.: 0 334 247 A2) discloses a kind of depressed cladding step refractive index profile fiber, the mode field diameters of which are respectively about 6 μm and 8 μm. The invention with the international patent application number of PCT/US2006/035894 and the publication number of WO 2007/040947 A1 presents a kind of bend insensitive fiber with the parabolic refractive index distribution. The American patent with the application number of US2008/0056654 A1 relates to a kind of three-cladding bend insensitive fiber with depressed refractive index of second cladding. The Chinese patent application with the application number of 200610051922.0 and the publication number of CN1971321A relates to a kind of ultra low water peak bend insensitive fiber.
The improvement of the bending performance of the fiber can also be started from the improvement of the coating resin layer of the fiber. For example, the Chinese patent application with the application number of 03124078.x and the publication number of CN1542473A has proposed a kind of fiber with special resin layer which has a high bending modulus. Another example, the coating process of Drake's ColorLock™ also enhances the microbending performance and reliability of the fiber.
The various prior arts have improved the bending performance of the fiber by improving the fiber structure, the fiber refractive index profile, the fiber coating resin layer and other different methods. However, fibers manufactured by using many of the methods as mentioned above are difficult to meet the requirements of the transmission performance specified in ITU.T G657. For some fibers that can meet the transmission requirements, the structures are relatively complex and the processes are very difficult and the manufacturing cost is very high. The problem to be solved by this invention is to find a new bending insensitive single mode fiber that has a relatively simple structure, relatively easy process and, at the same time, is also able to meet the standards of G.657 series of fiber proposed by the international ITU.T organization.